1. Field of the Invention
The present invention pertains to the removal of moisture from a gas; the invention is particularly suited for the removal of moisture required in the pretreatment of air for separation into its components via a cryogenic air separation process.
2. Prior Art
Processes for preparing a nitrogen-rich or oxygen-rich stream from a gas comprising nitrogen and oxygen and contaminated with water such as ambient air with the gas being passed to a pretreatment unit for the removal of moisture to avoid problems downstream are known. It is known that if water and carbon dioxide are not removed from the air, these components will freeze and block heat exchangers employed for cooling the gas prior to cryogenic distillation. There are essentially three commercial methods for removing water and carbon dioxide from a gas stream contaminated with water, possibly carbon dioxide and these include adsorption either by thermal swing adsorption (TSA) or pressure swing adsorption (PSA) and membranes for the removal of water, or, combinations of these three processes.
Representative patents which show the preparation of air streams utilizing pretreatment processes for the removal of water and/or carbon dioxide prior to recovery of at least the nitrogen or the oxygen component from a gas stream, including nitrogen or oxygen recovery by cryogenic processing are as follows:
U.S. Pat. No. 4,249,915 discloses a combination of adsorption systems initially utilizing pressure swing adsorption for the removal of moisture from air followed by thermal swing adsorption for the removal of carbon dioxide prior to cryogenic distillation of air into its components. Water is removed by pressure swing adsorption incorporating aluminas or alumino-silicate adsorbents, such as 5A and 13X zeolites, at pressures ranging from 3 to 20 bar. Thermal swing adsorption is carried out utilizing a 5A or 13X molecular sieve suited for selectively adsorbing carbon dioxide from a dilute CO.sub.2 containing gas stream. Temperatures range from about ambient temperature to 150.degree.C.
U.S. Pat. No. 4,931,070 discloses a process for the production of dry, high purity nitrogen utilizing a pretreatment system incorporating dual stage membrane removal of water followed by catalytically removing oxygen by combustion with hydrogen followed by residual removal of moisture utilizing a membrane dryer. The purge gas is used to enhance drying by passing the purge gas countercurrent to gas flow on the permeate side of the membrane drier unit. Representative membranes suited for water removal include those having a water/nitrogen separation factor of at least 50 preferably greater than 1,000 and are constructed of cellulose acetate, ethyl cellulose, silicon rubber, polyurethanes, polyamides and polysytrene.
U.S. Pat. No. 4,952,219 discloses a pretreatment process for producing high purity nitrogen by low temperature separation of air wherein the pretreatment process comprises passing feed air over a membrane selective to the permeation of water, then passing the partially dried air stream over a molecular sieve adsorbent to remove residual water and carbon dioxide and then introducing the moisture-free/carbon dioxide free stream to a low temperature separation unit. Representative membranes suited for the removal of moisture are comprised of polytrimethylsilylpropyne, polysulfones, polyamides and cellulose acetate.
U.S. Pat. No. 4,934,148 discloses a prepurifier adsorption system suited for a cryogenic air separation process incorporating a membrane drier. The prepurifier adsorption system is characterized by a membrane unit operated in a countercurrent flow path followed by cleanup of the moisture-free permeate in an alternating bed adsorption system. The membrane materials are impermeable barrier materials which include cellulose acetate, silcone rubber, etc. The adsorption systems which can be utilized include a pressure swing adsorption system suited for the removal of carbon dioxide and other components including residual water. Alternatively, a thermal swing adsorption cycle may be used wherein adsorption is carried out at a low temperature followed by desorption at an elevated temperature.
U.S. Pat. No. 4,701,187 discloses a gas separation technique involving the interaction between a membrane drying unit and an adsorption unit for providing an efficient process for producing a high purity product stream at recoveries greater than could be achieved by either unit operating individually. Membrane systems incorporating cellulose acetate, polyamides, silicone rubber, polyphenyleneoxide and the like are suggested as being as suited for the membrane drying units. Adsorption system contemplated include pressure swing, temperature swing and vacuum swing units utilizing a variety of molecular sieve adsorbents including, such as, carbon molecular sieves and zeolitic materials which include cation exchanged zeolites from the faujasite and mordenite families.
U.S. Pat. No. 4,690,695 discloses a process for the separation of gas incorporating a membrane dryer unit and a pressure swing adsorption unit. Materials suited for the membrane dryer unit component include hollow fibers of cellulose acetate, cellulose triacetate, and polysulfone. Zeolitic adsorbents are used in the pressure swing adsorption unit.